The semiconductor injection laser has gained a prominent position as the energy source for use in fibre optic communication systems. By selecting the appropriate semiconductor materials, conventionally III-V alloys, and through material processing and device fabrication techniques a range of laser diodes may be constructed which emit radiation in substantially the entire wavelength band of 0.8 to 1.7 .mu.m. Of particular interest are laser diodes which emit in the 1.3 to 1.55 .mu.m wavelength region as these are better matched to low attenuation fibre optic cables for high performance long distance transmission.
Diode lasers are conventionally fabricated in Fabry-Perot cavity designs or distributed feedback (DFB) configurations. The latter are better suited to optical communication systems in as much as they suffer less dispersion penalty due to their narrower optical spectrum.
Thermal considerations with respect to diode lasers have tended to concentrate on steady state issues such as mounting configurations, type of solder used and heat-sink materials. In long haul communication systems wherein accurate wavelength control is critical, however, wavelength shift due to transient temperature changes in the active region becomes an important consideration. Both Fabry-Perot and DFB type lasers are subject to changes in emission wavelength as a result of such temperature variations although DFB lasers are less prone to wavelength shift. This is because the shift in Fabry-Perot lasers follow the temperature dependence of the energy gap of the diode material which is greater than the temperature dependence of the refractive index, the influencing factor in DFB lasers. By way of example the typical wave length shift with temperature for a DFB laser may be as much as 0.08 nm per degree K while the shift for an Fabry-Perot type laser diode may be 0.6 nm per degree K. In any event, the emission wavelength of either type of laser will under go a shift to a lower or higher frequency as the device temperature increases or decreases respectively. From a practical point of view, a wavelength shift of approximately 1 Angstrom may be observed after a time of approximately 100 ns when the diode drive current to a DFB laser is increased by approximately 40 mA. This wavelength shift in combination with the inherent chromatic dispersion of a standard fibre causes difficulties in a fibre-optic-cable transmission system. For example, the shift in wavelength of a DFB laser operating at a nominal wavelength of 1.55 .mu.m can result in a variation in the delay of approximately 150 ps in the arrival time for a signal having travelled 100 km through a standard fibre.